In addition to the direct action of the ionic charge on water as a dielectric medium, ions may exert an influence on the equilibrium between the ice-like and non-ice-like forms which are present in room-temperature water. This provides a way of accounting for experimental results in a variety of areas, including entropy, heat capacity, temperature of maximum density, tracer self-difhsion, thermal conductivity, and dielectric relaxation, as well as viscosity and ionic mobility and their temperature coefficients. The tetrabutyl ammonium cation acts as a structure-promoter in the same way as do non-polar soIutes, amino acids and fatty acid anions. These various effects seem explicable in a straightforward manner in terms of a new picture of water as consisting of flickering clusters of hydrogen-bonded molecules, in which the co-operative nature of cluster formation and relaxation is related to the partially covalent character which is postulated for the hydrogen bond.Liquid water has long been known to possess distinctive structural features which are roughly describable by the statement that it retains a certain degree of similarity or analogy to ice. The amount of this " ice-like-ness " may be altered by changes in temperature and pressure and, as has also been known for a long time,l alterations which are presumably comparable (e.g., shifts in the temperature of maximum density) may also be evoked by the presence of ionic solutes. There is therefore nothing very new in inquiring into the ways in which such structural changes may influence, or may, in their turn, be studied by inferences from, observable thermodynamic and kinetic properties of ionic solutions. There have, however, been a number of advances in this field in recent times, and the subject is currently of some interest. In discussing it we must remember that we are trying to get at effects over and above those which the ions are expected to produce as charged spheres in a dielectric medium, even those connected with the discrete molecular nature of the medium, such as dipole saturation in the strong field near an ion. THE SIMPLE MODEL FOR SMALL IONSAmong the last-mentioned effects, which have no apparent necessary connection with the special character of water, is the immobilization of the dipolar solvent molecules which are nearest neighbours to the ions themselves.2 About a spherical ion of radius 2-3 8, in a medium of uniform dielectric constant 80, the field strength is of the order of 106 volts/cm and, except by Gurney,3 as noted below, it seems to be generally agreed that in aqueous solutions of ions not larger than Cs+ and I-the nearest-neighbour water molecules are always essentially immobilized by direct ion-dipole interaction. This idea and the related one of electrostriction have been invoked 4 to explain the small or negative values which salt solutions display of the solute partial molal volumes, heat capacities, compressibilities, etc. It also leads to a simple explanation of the influence of LiCl, 133

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NMR was used to study the structure and mobility of the perfluorosulfonate ionomer Nafion (Dupont trademark) in the presence of dimethyl methylphosphonate (DMMP). Fluorine-19 resonances arising from the ionomeric side chain narrowed rapidly with increasing DMMP content while the resonances arising from the perfluoroethylene backbone units remained broad. The plasticization of the side chains indicates that the DMMP is primarily associated with this structural unit which forms its own nanophase in Nafion. To further characterize the morphological structure, fluorine-19 spin diffusion experiments between the side chain and backbone resonances were performed. At low DMMP content, swelling of the pendant group domain is observed while at higher concentrations an unusual change in the spin diffusion indicates the development of a new morphology consisting of larger domains associated with the pendant group plus DMMP. To augment information on this system, proton pulse field gradient experiments were performed as a function of concentration and temperature to obtain DMMP self-diffusion constants. In the same concentration range where morphological change occurred, there was a rapid rise in translational mobility of the DMMP. This reflects the influence of the new larger domains which allow longer range motion of DMMP with fewer obstructions from the backbone domains which are relatively impermeable to DMMP. A fraction of the smaller domains originally present in dry Nafion remain even at high levels of DMMP, indicating an inability of all of the Nafion to undergo the morphological transition to the larger domains.

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